Engines include a cylinder head, a cylinder block, a crankcase, and an oil pan. With the foregoing construction, the crankcase serves as an engine unit case. Engines may include the cylinder head, the cylinder block, the crankcase, an upper oil pan, and a lower oil pan. With this construction, the crankcase and the upper oil pan serve as the engine unit case.
The engine unit case includes an oil pump for pressure feeding the oil to each portion in the engine for the lubrication and the cooling, and a regulator valve for adjusting the discharge pressure from the oil pump. An layout of the oil pump, the regulator valve, and oil paths (oil ports) in communication with the oil pump and the regulator valve impacts on reducing the size, reducing the weight, reducing the number of parts, and simplifying the machining and assembling of the engine unit case.
In case the regulator valve is assembled to the engine unit case, the relief oil discharged from the regulator valve drops in the oil pan provided at the bottom of the engine unit case. The oil hits the surface of the oil in the oil pan to generate the foam at the oil surface. With the foregoing construction, the air is likely to be mixed in the oil, which declines the lubrication performance at each portion of the engine.
In case the regulator valve is assembled to the engine unit case with a tightening means, it is required to provide an oil sealing member and a tightening member, or the like, at the fitting plane on the engine unit case for assembling the regulator valve. In other words, the construction of the engine unit case is complicated and the number of the parts increases. Further, because the regulator valve is projected from the fitting plane, the regulator valve is likely to be affected by the engine vibration, thus it is required to improve the vibration resistance thereof. In order to increase the vibration resistance performance, it is required to increase the thickness of the wall around the fitting plane or to provide a reinforcement rib for improving the fitting rigidity, which increases the weight of the engine unit case, and thus to increase the weight of the engine per se.
JPH05(1993)-10721U discloses the construction that an oil pipe is connected to an oil discharge port of the regulator valve so that the relief oil is directly returned to the oil pan in order to prevent the air mixture in the oil. However, with the foregoing construction, the number of parts for the engine per se is increased. JPH08(1996)-484Y2 discloses the construction for covering the surroundings of the discharge port of the relief oil of the regulator valve with a division wall provided either at the engine block (i.e., crankcase) or the oil pan. Although the air mixture in the oil can be avoided to some extent, the air mixture cannot be completely prevented because the foregoing structure is the construction for discharging the relief oil from the regulator valve to the external space. JPH10(1998)-141039A discloses the construction that includes the regulator valve positioned between mating faces of the engine block and the oil pan, and the relief oil is directly returned to a suction passage of the oil pump without being through the external space. The foregoing construction requires the complicated oil path (port) for both the engine block and the oil pan and complicated oil-sealing member at joining surfaces between the engine block and the port and between the oil pan and the port.
A need thus exists for an engine unit case, which prevents the air mixture in a relief oil discharged from a regulator valve.